Now there are two practical ways by which continuous waves can be set up for sending either telegraphic signals or telephonic speech and music and these are with: (a) an oscillation arc lamp, and (b) a vacuum tube oscillator. The oscillation arc was the earliest known way of setting up sustained oscillations, and it is now largely used for commercial high power, long distance work. But since the vacuum tube has been developed to a high degree of efficiency and is the scheme that is now in vogue for amateur stations we shall confine our efforts here to explaining the apparatus necessary and how to wire the various parts together to produce several sizes of vacuum tube telegraph transmitters.
Sources of Current for Telegraph Transmitting Sets.--Differing from a spark-gap transmitter you cannot get any appreciable results with a low voltage battery current to start with. For a purely experimental vacuum tube telegraph transmitter you can use enough B batteries to operate it but the current strength of these drops so fact when they are in use, that they are not at all satisfactory for the work.
You can, however, use 110 volt direct current from a lighting circuit as your initial source of power to energize the plate of the vacuum tube oscillator of your experimental transmitter. Where you have a 110 volt direct current lighting service in your home and you want a higher voltage for your plate, you will then have to use a motor-generator set and this costs money. If you have 110 volt alternating current lighting service at hand your troubles are over so far as cost is concerned for you can step it up to any voltage you want with a power transformer. In this chapter will be shown how to use a direct current for your source of initial power and in the next chapter how to use an alternating current for the initial power.
An Experimental Continuous Wave Telegraph Transmitter.--You will remember that in [Chapter XV] we learned how the heterodyne receiver works and that in the separate heterodyne receiving set the second vacuum tube is used solely to set up oscillations. Now while this extra tube is used as a generator of oscillations these are, of course, very weak and hence a detector tube cannot be used to generate oscillations that are useful for other purposes than heterodyne receptors and measurements.
There is a vacuum tube amplifier [Footnote: This is the radiation UV-201, made by the Radio Corporation of America, Woolworth Bldg., New York City.] made that will stand a plate potential of 100 volts, and this can be used as a generator of oscillations by energizing it with a 110 volt direct current from your lighting service. Or in a pinch you can use five standard B batteries to develop the plate voltage, but these will soon run down. But whatever you do, never use a current from a lighting circuit on a tube of any kind that has a rated plate potential of less than 100 volts.
The Apparatus You Need.--For this experimental continuous wave telegraph transmitter get the following pieces of apparatus: (1) one single coil tuner with three clips; (2) one .002 mfd. fixed condenser; (3) three .001 mfd. condensers; (4) one adjustable grid leak; (5) one hot-wire ammeter; (6) one buzzer; (7) one dry cell; (8) one telegraph key; (9) one 100 volt plate vacuum tube amplifier; (10) one 6 volt storage battery; (11) one rheostat; (12) one oscillation choke coil; (13) one panel cut-out with a single-throw, double-pole switch, and a pair of fuse sockets on it.
The Tuning Coil.--You can either make this tuning coil or buy one. To make it get two disks of wood 3/4-inch thick and 5 inches in diameter and four strips of hard wood, or better, hard rubber or composition strips, such as bakelite, 1/2-inch thick, 1 inch wide and 5-3/4 inches long, and screw them to the disks as shown at A in Fig. 75. Now wrap on this form about 25 turns of No. 8 or 10, Brown and Sharpe gauge, bare copper wire with a space of 1/8-inch between each turn. Get three of the smallest size terminal clips, see B, and clip them on to the different turns, when your tuning coil is ready for use. You can buy a coil of this kind for $4.00 or $5.00.
The Condensers.--For the aerial series condenser get one that has a capacitance of .002 mfd. and that will stand a potential of 3,000 volts. [Footnote: The U C-1014 Faradon condenser made by the Radio Corporation of America will serve the purpose.] It is shown at C. The other three condensers, see D, are also of the fixed type and may have a capacitance of .001 mfd.; [Footnote: List No. 266; fixed receiving condenser, sold by the Manhattan Electrical Supply Co.] the blocking condenser should preferably have a capacitance of 1/2 a mfd. In these condensers the leaves of the sheet metal are embedded in composition. The aerial condenser will cost you $2.00 and the others 75 cents each.
